A predominant portion of the energy consumed by the chemical industry which assumes the principal role in producing substances is used not for chemical reactions but for the separation or purification of products, especially for handling aqueous solutions which are great in specific heat and require great energy for heating or cooling. For industries wherein improved productivities and savings in energy are of the greatest importance, what matters is how to conduct efficient separation or purification. Attention has been directed to the use of membranes as useful means for carrying out efficient separation or purification almost without necessitating heating or cooling. It has been desired to develop membranes having various characteristics for use in novel processes, while it is an important problem to provide membranes of improved characteristics for use in existing processes.
Efforts to develop membrane materials are directed to the investigation of materials which can be made into a membrane structure having a uniform distribution of pores suited to the contemplated purpose and also materials which can be made into a thin membrane retaining the desired strength to achieve a high permeability. However, it is difficult to prepare membranes which are uniform in pore size distribution.
The separation or purification of substances is achieved essentially by the selective migration of the substance, and the efficiency of the membrane process is dependent on the ease with which the substance ingresses into and egresses from the membrane and moves through the membrane, so that the affinity of the membrane for the substance to be separated off, the migration of the substance into the membrane and the diffusibility thereof through the membrane are important factors. The ideal condition for the affinity and the migration of the substance into the membrane is the compatibility between the substance and the membrane material, permitting each to dissolve in the other. Further to assure satisfactory diffusibility, the molecules must retain flexibility even at low temperatures, while permitting the membrane to retain its shape at all times. Thus, it is required for developing membranes to search for a material which can be made into a membrane structure having a uniform distribution of pores suited to the intended purpose and also for a material which can be made into a thin membrane of desired strength to achieve a high permeability.
Since the 1960's intensive research has been carried out on polyphosphazenes by H. R. Allcock et al. For example, Inorganic Chemistry, Vol. 5, No. 10, p. 1709 (1966), etc. disclose that such a compound can be prepared by subjecting hexachlorotriphosphonitrile to ring-opening polymerization and to alkoxylation as represented by the following scheme, while it is also known that similar reactions can be carried out using amines. ##STR2##
Many polyphosphazene compounds have been synthesized by these methods. We have directed attention to the flexible characteristics of the phosphazene skeleton represented by many items of data accumulated as to the properties and conceived utilization of the characteristics.
On the other hand, secondary batteries presently in wide use include lead batteries and nickel-cadmium batteries wherein the single-cell voltage is about 2 V, and an aqueous solution is used. In recent years, efforts are made in investigate and develop secondary batteries of high energy density which give a high single-cell voltage of at least 3 V and include a negative electrode of lithium. However, when lithium is used which reacts with water or the like, aprotic electrolytes must be used since aqueous electrolytes are not usable. Although polar organic solvents are presently in wide use, a majority of these solvents have a low boiling point (high vapor pressure) are inflammable and therefore involve the likelihood of staining neighboring members and ignition or firing due to a leak or break and the hazard of explosion due to erroneous use or overcharging. Furthermore, repeated discharge and charge of the secondary battery as contemplated form dendrite on the negative electrode, entailing the problem of reduced discharge-charge efficiency and short-circuiting between the positive and negative electrodes. Accordingly, many reports are made on the development of techniques for improving the discharge-charge efficiency of the negative electrode and the cycle life by inhibiting dendrite. Proposed in these reports are, for example, use of a methylated cyclic ether solvent as the solvent for battery electrolytes (K. H. Abraham et al. in "Lithium Batteries," J. P. Gabano, editor, Academic Press, London (1983)), a method of forming an ionically conductive protective film at the Li interface by adding polyethylene glycol, polypropylene glycol, polyethylene oxide or like additive to an electrolyte system (Journal of Power Sources, Vol 12, No. 2, pp. 83-144 (1984) and Unexamined Japanese Patent Publication No. SHO 60-41773), a method of inhibiting Li dendrite by allowing an electrode per se with Al (Unexamined Japanese Patent Publication No. SHO 59-108281).
On the other hand, M. Armand and N. Duclot disclose a novel secondary battery of high energy density incorporating a thin-film polymer electrolyte in Laid-Open French Patent Publication No. 2442512 and European Patent No. 13199. Yao et al. (J. Inorg. Nucl. Chem., 1967, 29, 2453) and Farrington et al. (Science, 1979, 204, 1371) generally describe inorganic ionically conductive solids. Much attention has been directed to these solids from the viewpoint of basic research and because of their use as electrolytes for batteries of high energy density and for sensors. Sequlir et al. (Extended Abstracts, 163rd Meeting Electrochemical Society, 1983, 83, 751, Abstract, No. 493) describe a battery of novel design including a solvent-free thin-film polymer electrolyte, stating that the electrolyte is usable at a medium temperature of about 100.degree. C. as determined by testing.
P. M. Blonsky et al. (J. Am. Chem. Soc., 106, 6854, 1984) state that polyphosphazene (MEEP) is useful as an electrolyte for electrochemical batteries. However, they merely disclose data as to a.c. conductivity in the range of from 30.degree. C. to 97.degree. C. and have not effected discharge and charge with d.c.
As described above, in developing high-performance secondary batteries as heretofore attempted, it is critical to avoid the formation of dendrite, leakage, ignition, firing, explosion and like hazards. It has been desired to complete a secondary battery which has a single-cell voltage of at least 3 V, is assured of safety against these hazards and is operable at the usual ambient temperature.
An object of the present invention is to provide a material which can be made into a membrane having high affinity for the substance to be separated off, excellent flexibility, desired strength despite its small thickness and high permeability.
Another object of the invention is to provide a high-performance secondary battery which is substantially free of formation of dendrite and leakage, has no ignitability due to the features of its frame retardancy and low vapor pressure and is assured of high safety against explosion or the like.